Generally speaking, a crankcase is a tight casing which protects, within a closed volume, mechanical elements imparted with movements (such as transmissions, gearboxes, motors, etc . . . ) as well as a lubricant (such as oil, . . . ).
The temperature which prevails inside the crankcase enclosure is relatively high because of the heat released as a result of combustion, heat-conduction, mechanical friction and other phenomena.
This often creates thick mists constituted of gaseous mixtures of air and vaporized oil which must not escape into the atmosphere for fear of polluting the environment and of increasing the consumption of oil.
Concomitantly, the crankcase must be depressurized by opening the enclosure to the atmosphere, so as to avoid the problems linked to possible pressurization caused by the aforesaid overheating. Moreover, if the pressure reduces inside the enclosure due to possible cooling or to a special dynamical effect, this creates an external draught containing particles and an internal ventilation the effect of which can prove damaging to the mechanisms housed inside the crankcase (corrosion, . . . ).
Devices have already been developed, which comprise a housing provided with vents which create a communication with the outside and containing a filter cartridge.
In such devices, the communication between the crankcase and the outside is made through a vertical passageway provided between the crankcase and the housing, through the porous wall of the filter cartridge and through the vents.
In this case, the air goes through the filter cartridge in both directions and in the same manner. Said filter cartridge is generally of paper type. Any stoppage of the droplets of oil mists is virtually inexistant because the phenomenon of coalescence, which consists in regrouping the micro-droplets into large enough drops which can be stopped by gravity from being driven by the air stream, occurs only partly during the passage through the filtering agent. A large part of the oil is thus expelled out of the crankcase through the cartridge, without being recovered.
Also, in the known devices, the vents are generally situated in the lower part of the housing to allow the exhaust towards the outside of any oil drops which have formed in the filtering agent.
Finally, in the event of the cartridge clogging-up, a great difference of pressure can set up between the outside and the inside of the crankcase enclosure and cause breakages or tightness failures.
In the case of a liquid-containing reservoir topped with an air lock, the changes of temperature, pressure or level of the liquid, require in many cases, that the upper part of the reservoir be in communication with the outside. With some easily vaporizable liquids, leaks of liquid mists may occur and pollute the environment. Also, it is often necessary to filter the air entering the reservoir so that it does not contaminate the liquid. The problem to be solved with such reservoirs is the same as that of recycling the oil mists in a crankcase. Affected reservoirs can be liquid storage reservoirs or fermentation reservoirs. For clarity's sake, the only reservoirs referred to hereinafter, are the crankcases with oil sumps.